In a liquid processing technique for processing objects to be processed such as semiconductor wafers by supplying thereto a process liquid, it is generally important to monitor an amount of the process liquid stored in a tank.
The amount of the process liquid stored in the tank has been usually detected by a liquid level sensor. For example, a liquid level detector has been known in the conventional art (e.g., JP Patent No. 3349862) including: a liquid level measuring pipe having one end connected to a tank storing a process liquid, and the other end disposed on a position higher than the liquid level of the process liquid in the tank; a liquid level sensor for detecting a position of the liquid level of the process liquid stored in the tank by means of the process liquid fed to the liquid level measuring pipe from the tank; and a bubble removing pipe for removing bubbles, having one end connected to the liquid level measuring pipe at a position lower than the liquid level of the process liquid in the tank, and the other end opened at a position higher than the liquid level of the process liquid in the tank.
In the art described in JP Patent No. 3349862, the liquid level measuring pipe and the bubble removing pipe are connected to each other via a horizontal pipe connected to a side part of the tank. Suppose that bubbles are generated when the process liquid in the tank is discharged, and the bubbles remain in a part (i.e., the horizontal pipe) where the liquid level measuring pipe and the bubble removing pipe are connected to each other. Then, in some cases, when the new process liquid is supplied into the tank after the used process liquid is discharged, there is a possibility that the bubbles still remaining in the connecting part (horizontal pipe) enter the liquid level measuring pipe so as to invite a malfunction of the liquid level sensor.
In particular, when a hydrogen peroxide solution (H2O2) containing a surface active agent is used as a process liquid, larger bubbles are likely to be generated. If such larger bubbles enter the liquid level measuring pipe, the amount of the process liquid in the tank may not be accurately detected.
The present invention has been made in view of the above circumstances. The object of the present invention is to provide a liquid level detector capable of accurately detecting and monitoring an amount of a liquid stored in a tank, without suffering from an influence of bubbles generated in the liquid, and to provide a liquid processing system provided with the liquid level detector.